Current mode DVR or PVCOM with integrated impedances

ABSTRACT

One or more resistors or resistances are integrated in a 7-bit DVR or PVCOM integrated circuit. A 7-bit DVR or PVCOM integrated circuit includes a 7-bit DAC. The integrated resistors or resistances (R1, R2, or RSET, or any combination) reduces the number of external components, reduces the number of pins, and increases the accuracy of the DVR or PVCOM circuit. The least significant bit (LSB) of the DAC depends only on ratios of internal resistors, which can be made very accurate and independent of temperature.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation of U.S. patent applicationSer. No. 14/918,299, filed Oct. 20, 2015, issued as U.S. Pat. No.9,548,723 on Jan. 17, 2017, which is a continuation of U.S. patentapplication Ser. No. 14/685,532, filed Apr. 13, 2015, issued as U.S.Pat. No. 9,166,566 on Oct. 20, 2015, which is a continuation of U.S.patent application Ser. No. 13/783,174, filed Mar. 1, 2013, issued asU.S. Pat. No. 9,007,098 on Apr. 14, 2015. These applications areincorporated by reference along with all other references cited in thisapplication.

BACKGROUND OF THE INVENTION

The invention is related to the field of electrical circuits and morespecifically to driver circuitry for display panel products.

Electronic visual displays are used in a wide range of applicationsincluding computer monitors, televisions, instrument panels, aircraftcockpit displays, and signage. They are common in consumer devices suchas laptop computers, video players, music players, gaming devices,clocks, watches, calculators, telephones, smartphones, tablets, and manyother devices.

Some examples of display panel technologies include liquid crystaldisplays (LCDs), organic led emitting diode (OLED) displays, and plasmadisplays. Such displays operate according to various principles. Forexample, LCDs use the light modulating properties of liquid crystals toproduce images. Since LCDs do not emit light, there is often a backlightbehind the LCD panel to illuminate the display. Other displaytechnologies work according to different principles.

Electronics are used to drive an electronic display. These electronicsprovide power and electrical input. For example, there are voltages forthe row and column drivers to drive a thin-film transistor (TFT) LCD.Electronics generate voltage waveforms to achieve (1) color outputstability to alleviate flickering and inconsistent color, and (2) liquidcrystal stability to prevent display damage due to localized net voltagebuild-up.

Further, a LCD display panel has a VCOM input. VCOM is adjusted to matchthe capacitance and performance specifications of the TFT panel tomaximize contrast and minimize flickering. The VCOM can be aprogrammable function, which can be used to adjust a panel to maximizecontrast, minimize flickering during operation, and optimize panelperformance.

It is desirable to improve electronics used to drive electronic visualdisplays, so that these displays and the electronics used to drive themto improve performance, reduced cost, and reduce power consumption.Therefore, improved electronics and circuits are needed.

BRIEF SUMMARY OF THE INVENTION

A current mode digitally variable resistor (DVR) or programmable VCOM(PVCOM) circuit, or both, has integrated resistors, a programmablecurrent setting resistor, and improved accuracy. One or more resistorsor resistances are integrated in a 7-bit DVR or PVCOM integratedcircuit. A 7-bit DVR or PVCOM integrated circuit includes a 7-bit DAC.The integrated resistors or resistances (R1, R2, or RSET, or anycombination) reduces the number of external components, reduces thenumber of pins, and increases the accuracy of the DVR or PVCOM circuit.The least significant bit (LSB) of the DAC depends only on ratios ofinternal resistors, which can be made very accurate and independent oftemperature.

In an implementation, to provide flexibility to the customer, thecurrent setting resistor for the DAC is programmable. For example, thecustomer can choose between several different values, maintaining theflexibility of the external approach. The 7-bit DVR or PVCOM solutionwith integrated resistors or resistances is compatible with the machinesused currently in production for LCD panels.

In a specific implementation, a device includes: a 7-bitdigital-to-analog (DAC) converter circuit formed on a semiconductorsubstrate of an integrated circuit; a first operational amplifiercircuit, formed on the semiconductor substrate, comprising a first inputconnected to the 7-bit digital-to-analog converter circuit; and a secondoperational amplifier circuit, formed on the semiconductor substrate,comprising a first input connected to an output of the secondoperational amplifier circuit and a voltage output of the integratedcircuit.

The device further includes: a first internal resistance, formed on thesemiconductor substrate, connected between a first supply voltage and avoltage bias node; a second internal resistance, formed on thesemiconductor substrate, connected between the voltage bias node and asecond supply voltage line, wherein the voltage bias node is connectedto a second input of the second operational amplifier circuit; atransistor, formed on the semiconductor substrate, connected between thevoltage bias line and a second input of the first operational amplifiercircuit; and a third internal resistance, formed on the semiconductorsubstrate, connected between the second input of the first operationalamplifier circuit and the second supply voltage line.

In various implementations, the third internal resistance iselectrically programmable in a range from about 1000 ohms to about 20kiloohms. The first internal resistance has a value of about 50kiloohms. The first internal resistance has a value of about 100kiloohms. The first supply voltage has a value about 12 volts. Thefirst, second, or third internal resistances, or any combination, areprogrammable to have a plurality of resistance values

A connection to the third internal resistance is not made available viaan external pin to the integrated circuit. A connection to the secondinput node of the second operational amplifier circuit is not madeavailable via an external pin to the integrated circuit. A connection toan output of the 7-bit digital-to-analog converter circuit is not madeavailable via an external pin to the integrated circuit.

The first input of the first operational amplifier circuit is a positiveinput. The first input of the second operational amplifier circuit is anegative input.

In a specific implementation, a method includes: forming a 7-bitdigital-to-analog (DAC) converter circuit on a semiconductor substrateof an integrated circuit; forming a first operational amplifier circuiton the semiconductor substrate, the first operational amplifierincluding a first input coupled to the 7-bit digital-to-analog convertercircuit; forming a second operational amplifier circuit on thesemiconductor substrate, the second operational amplifier including afirst input connected to an output of the second operational amplifiercircuit and a voltage output of the integrated circuit; forming a firstinternal resistance on the semiconductor substrate; and connecting thefirst internal resistance between a first supply voltage and a voltagebias node.

The method further includes: forming a second internal resistance on thesemiconductor substrate; connecting the second internal resistancebetween the voltage bias node and a second supply voltage line;connecting the voltage bias node a second input of the secondoperational amplifier circuit; forming a transistor on the semiconductorsubstrate; connecting the transistor between the voltage bias line and asecond input of the first operational amplifier circuit; forming a thirdinternal resistance on the semiconductor substrate; and connecting thethird internal resistance between the second input of the firstoperational amplifier circuit and the second supply voltage line.

In various implementations, the method can includes: electricallyprogramming the first internal resistance to have a first value;electrically programming the second internal resistance to have a secondvalue; and electrically programming the third internal resistance tohave a third value. The first value is about 50 kiloohms, second valueis about 100 kiloohms (e.g., more than the first value, or at leastabout twice the first value), and third value is about 5 kiloohms (e.g.,less than the first value, or about one-tenth of the first value). As anexample, the first internal resistance can have a value of at least 1000ohms.

Other objects, features, and advantages of the present invention willbecome apparent upon consideration of the following detailed descriptionand the accompanying drawings, in which like reference designationsrepresent like features throughout the figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a block diagram of a display system.

FIG. 1B shows a circuit diagram for a single LCD pixel.

FIG. 2 shows a circuit diagram of a programmable VCOM driver circuitwith one or more internal resistors or resistances.

FIGS. 3A-3C show an equation and tables of operating conditions of theprogrammable VCOM driver circuit with one or more internal resistors orresistances.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1A shows a block diagram display system. This display system can beincorporated in computer monitors, televisions, instrument panels,aircraft cockpit displays, signage, laptop computers, video players,music players (e.g., Apple's iPod product family), gaming devices,cameras, clocks, watches, calculators, telephones, smartphones (e.g.,Apple's iPhone product family, Google's Nexus product family, Samsung'sGalaxy product family), tablets (e.g., Apple's iPad product family,Google's Nexus product family, or Samsung's Galaxy product family), andmany other devices.

The system includes a display controller 103 that drives a display panel105 and a voltage driver generator and a voltage drive generator 107,which also drives the display panel. The voltage drive generator cangenerates a reference voltage for the display panel.

The system can be an LCD display system, such as an active matrixthin-film transistor (TFT). FIG. 1B shows a circuit diagram for a singleLCD pixel. Numerous pixels are arranged in an array to form a displaypanel. In an implementation, the voltage drive generator generates theVCOM reference voltage for pixels of the display panel.

Some common resolutions for panels includes 7680 by 4320 (e.g., 8K),4096 by 2304 (e.g., 4K), 3840 by 2160 (e.g., 4K UHD), 2800 by 1800, 2560by 1200, 2560 by 1400, 1600 by 1200, 1920 by 1080 (e.g., HD 1080), 1280by 720 (e.g., 720p), 1136 by 640 (e.g., iPhone 5), 1280 by 768, 960 by640 (e.g., iPhone 4S), 1024 by 768, 800 by 600, 800 by 480, 640 by 480,480 by 320, and many more.

The LCD is panel includes glass, TFT array substrate, liquid crystal,polarizer, color filters, and other components to implement a TFT LCD.The drive electronics of a TFT activate the TFT array substrate,resulting in an induced electromagnetic field that affects the liquidcrystal. The liquid crystal is twisted in response to the inducedelectric field, allowing light to shine through the glass sandwich. Thelight is modulated by the color filters to output the desired color.

In other implementations, the display panel can be of another LCDtechnology such as a passive-matrix LCD, super-twisted nematic (STN),double-layer STN (DSTN), or color-STN (CSTN). Or the display panel canuse organic light emitted diode (OLED). Aspects of the invention can beapplied to various display panel technologies.

A VCOM circuit outputs a VCOM voltage reference, typically for an LCDscreen. LCD screens have an array of pixels constantly lit by abacklight. The constancy of the light removes the type of flickerusually associated with cathode ray tube (CRT) screens (phosphorspulsing with each refresh cycle). Instead, an LCD pixel has upper andlower plates with grooves cut perpendicular to each other as in. Thesegrooves align the crystals to form channels for the backlight to passthrough to the front of the panel. The amount of light emitted dependsupon the orientation of the liquid crystals and is proportional to theapplied voltage.

Referring to FIG. 1B, the gate voltage acts as a switch and is commonlyamplified to become −5 volts to 20 volts. The video source, typicallyranging from 0 volts and 10 volts, provides the intensity informationthat appears across the pixel. The bottom of the pixel is commonlyconnected to the backplane of the panel. The voltage at this node isVCOM.

While this set-up is functional, it reduces panel lifetime. Assuming theVCOM voltage is at ground, the voltage across the pixel varies from 0volts to 10 volts. Assuming an average of 5 volts, there is substantialDC voltage across each pixel. This DC voltage causes charge storage, ormemory. In the long term, it is a form of aging, degrading the pixels byelectroplating ion impurities onto one of the electrodes of the pixel.This contributes to image retention, commonly known as a sticking image.

The construction of the LCD panel is generally symmetrical and either apositive or a negative voltage can be used to align the crystals. Atechnique is to adjust the common voltage (VCOM) to a midpoint of thevideo signal (e.g., 10/2, which is 5 volts) or other desired voltagelevel (e.g., ⅔, ¾, or other percentage of the maximum signal voltage).Now the video signal swings above and below the common voltage (VCOM),creating a net zero effect on the pixel. This net zero effect on theliquid crystal eliminates the aging and image retention issues. Atradeoff for this technique is resolution, since the video signaltravels 5 volts to full brightness instead of the entire 10-volt range.

The VCOM voltage should be set very precisely (e.g., midpoint or ½ ofthe video signal, ⅔ of signal voltage, or other value) to avoid flicker.To illustrate why a panel will flicker, let's assume that due tomanufacturing of the panel the VCOM is 5.5 volts. If the video signalswings between 0 volts and 10 volts, the full-scale voltage will bedifferent on each field. On one field, the full-scale voltage will be4.5 volts and on the other, the full-scale voltage will be 5.5 volts.This difference in full-scale voltage translates to a difference inintensity, experienced as flicker.

Due to the variations in construction of each panel, the optimal VCOMvoltage can differ from panel to panel or across a single panel. It isimportant to be able to set the VCOM precisely and also be able tochange it as needed to work optimally with a particular panel.

In the specific implementation shown, the display controller and voltagedrive generator are circuits residing on separate integrated circuits ordifferent semiconductor substrates. But in other implementation, some orall components of the voltage drive generator can be incorporated intothe drive controller integrated circuit (or alternatively, integratedwithin the display panel).

FIG. 2 shows a VCOM generator and calibrator circuit. This circuitry canbe part of the display system of figure, such as in the voltage drivegenerator (or display controller). The circuit has SDA 7 and SCL 8inputs, a VCOM output 27, and a power input AVDD 42. SDA and SCL inputsconnect to an I2C control interface. The I2C control interface connectsto an EEPROM (electrically erasable program read only memory) which is anonvolatile memory storage. The I2C control interface connects via a7-bit bus to a 7-bit digital-to-analog converter (DAC).

The I2C bus interface (or Inter-Integrated Circuit) is a multi-masterserial single-ended computer bus developed by Philips that is used toattach low-speed peripherals to a motherboard, embedded system,cellphone, or other electronic device. The I2C specification can befound at the NXP Semiconductor Web site and documented in PhilipsSemiconductors, “The I2C-Bus Specification, Version 2.1,” January 2000(document order no. 9398 383 40011). Documents on the I2C interface areincorporated by reference.

Although a specific implementation has a 7-bit DAC, otherimplementations can use a DAC have less than 7 bits (e.g., 1, 2, 3, 4,5, or 6 bits) or more than 7 bits (e.g., 8, 10, 12, 14, 16, 18, 20, 24,32, or other number of bits). AVDD connects through a resistor orresistance 19R to provide power the DAC. The DAC has an internalresistor or resistance R that is connected to a first operationalamplifier or op amp.

AVDD supplies power to the first op amp (not shown) and to a second opamp. VCOM is connected to a negative (−) input of the second op amp andalso an output of the second op amp. A positive (+) input of the secondop amp is connected to a bias voltage generated by a divider ofresistors or resistances R1 and R2. This bias voltage may also bereferred to as a DVR output. R2 is also connected to ground. A nodebetween R1 and R2 is connected to a drain node of a transistor, whichhas a current IRSET passing through it. A source node of the transistoris connected to a negative (−) input of the first op amp and to aresistor or resistance RSET. RSET is also connected to ground.

In an implementation, the transistor is an n-channel or NMOS transistor,but in other implementation, the transistor can be a p-channel or PMOStransistor.

Some existing digitally variable resistors (DVR), VCOM generators, andprogrammable VCOM (PVCOM) integrated circuits include Integrated MemoryLogic's (IML) iML7976 and iML7972. Data sheets, white papers, and otherpublic documentation (such as available as the IML Web site) on theseproducts are incorporated by reference. The circuitry described in thispatent application can be used in such DVR or PVCOM products.

In an implementation, 7-bit current-mode DVR and PVCOM circuits usethree external resistors: two resistors (e.g., R1 and R2) to set aninitial point or bias voltage for VCOM and one resistor (e.g., RSET) setcurrent for DAC least significant bit (LSB). The PVCOM is programmablevia the DAC, which can adjust its output. By adjusting the DAC output(7-bit precision), the voltage is varied across the RSET resistor. Thisis used to in production to control the quality of the panel. Alsoexisting 7-bit DVR and PVCOMs need accurate external resistor forcurrent setting, which is expensive.

Current 10-bit DVR and PVCOM integrated circuits (ICs) are notcompatible with the existing production equipment. The DVR is used toadjust the LCD panel, on a panel to panel base. Each single panel isadjusted in production, looking for best screen performance (e.g., lowflicker). The machines used in the factory have a 7-bit I2C interfaceprotocol. It is not practical for customers to change the productionmachines. The setting of the resistors is made off-line (IC level) andis the same for all the LCD panels (at least for one model or size). Soin production, on the finished board level, the customer only needs toadjust the 7-bit DAC for optimal screen accuracy.

In an implementation, resistors or resistances R1, R2, and RSET arefabricated on the same integrated circuit as the other components of theDVR or PVCOM. External resistors are not used. By integrating theresistors in the 7-bit DVR or PVCOM it is possible to reduce the numberof external components, reduce the number of pins, while increasing theaccuracy of the DVR or PVCOM. When the actual LSB depends only on ratiosof internal resistors, it can be made very accurate and independent oftemperature.

The internal resistors or resistances can be formed using a techniqueused to create a resistance within an integrated circuit, such as usingpassive or active components, or a combination of these. For example,the resistance can be formed using a track of resistive material (e.g.,polysilicon, diffusion, or a combination of materials chained together),transistor, or diode. As internal resistors or resistances, these areformed on the same integrated circuit or semiconductor substrate asother components such as the DAC, operational amplifiers, andtransistor.

To provide flexibility to the customer, the current setting resistor forthe DAC is programmable. The customer can choose between severaldifferent values, maintaining the flexibility of the external approach.This implementation is compatible with the machines used currently inproduction for LCD panels.

Programmable resistors or resistances can be provided by way ofcircuitry that alters resistance values based on memory or storageelements such as programmable fuses, links, antifuse, flip flops, memorycells (e.g., RAM, SRAM, PROM, EPROM, or EEPROM), and many others.Depending on the programming of the memory or storage elements, theresistor or resistors (R1, R2, and RSET) can be varied. The resistancescan be selected by electrically programming the device (e.g., by usinghigh voltages or a voltage or programming sequence to select aresistance value).

R1 and R2 are typically about equal (so that a starting voltage of VCOMis about 50 percent of AVDD), but these resistor can have different. Ina specific implementation, the resistor divider is set to ⅔ or 66percent of AVDD (i.e., R2 is 2*R1). The typically value of theresistances is typically in the order of several kiloohms. It is definedby a design trade off between low current consumption and noiseimmunity. A specific design uses about 50 kiloohms and 100 kiloohms.Other design use combinations of 10 and 20 kiloohms, 20 and 40 kiloohms,25 and 50 kiloohms, 30 and 60 kiloohms, 60 and 120 kiloohms, 75 and 150kiloohms, and other combinations. Other specific designs use R1 equal toR2 with a value of 10 kiloohms, 20 kiloohms, 30 kiloohms, 50 kiloohms,100 kiloohms, or other values.

The RSET range is typically between 1 kiloohm and 20 kiloohms (FIG. 3C).The value is selected by the customer depending if they want accuracy(e.g., RSET will be selected to a relatively higher value) or a quickconvergence to the desirable point (RSET will be selected to be arelatively lower value).

In an implementation, the values of the integrated resistors areprogrammable to have a value as discussed above. For example, R1 isprogrammable to be any one of 10, 20, 30, 40, 50, 60, or 100 kiloohms,or any combination of these. R2 is programmable to be any one of 10, 20,30, 40, 50, 60, or 100 kiloohms, or any combination of these. RSET isprogrammable to be any one of 1, 3, 5, 7.5, 10, 12.5, 15, or 20kiloohms, or any combination.

R1 and R2 are programmable that they can have an R1/R2 ratio that canchange between about ½ to about 2 (which would occur in very specificcircumstances). In other implementations, the R1/R2 ratio isprogrammable to vary between about ½ to about 1. RSET programmabilityvaries between about 1000 ohms to about 20 kiloohms.

Some benefits include of integrated resistors include: reduction ofnumber of external components in DVR and PVCOM (three component less);reduction of number of pins in DVR and PVCOM (potentially down to onepin, a VCOM output); improvement of accuracy (depending only on ratio ofresistors, i.e., very accurate in CMOS processes); increased flexibility(use of programmable resistor for current setting); and backwardcompatible (can use existing production tools and provide sameperformance).

Three resistors (R1, R2, and RSET) are integrated into the integratedcircuit, so that external resistors are not used. By doing so the numberof external components is reduced, and so is the cost. This also allowsto reduce the number of pins of the integrated circuit, because threepins can be eliminated: (i) connection to RSET, (ii) connection forpositive input of op amp (typically connected externally to R1 and R2),and (iii) output of DVR (typically connected externally to the positiveinput of the op amp).

In various other implementations, only one or any combination ofresistors R1, R2, and RSET can be integrated in the integrated circuit.For example, only R1 is integrated into the integrated circuit and R2and RSET remain external to the integrated circuit. Only R2 isintegrated into the integrated circuit and R1 and RSET remain externalto the integrated circuit. Only RSET is integrated into the integratedcircuit and R1 and R2 remain external to the integrated circuit. Infurther implementations, R1 and R2 are integrated while RSET isexternal. R1 and RSET are integrated while R2 is external. R2 and RSETare integrated while R1 is external.

Depending on the number of resistors integrated, the number of pins thatare reduced by integrated one or more resistors R1, R2, and RSET can beup to three, fewer than three, or more than three.

Further, the accuracy of the system is not affected, but rather improvedby use of internal resistors. Since the final voltage depends only onratio of internal resistors, the ratio can be made very precisely and isinherently independent from temperature.

In FIGS. 3A-3C, see equation and tables showing operating conditions forthe circuitry. FIG. 3A gives an equation of a relationship between AVDD,R1, R2, and RSET. In a specific implementation (refer to FIG. 3B), AVDDis about 12.2 volts, R1 is about 50 kiloohms, R2 is about 100 kiloohms,and VCOM (max) is about 8.1 volts, which is about 0.677 of AVDD. Adefault value for RSET can be about 5 kiloohms. For other values ofRSET, values are provided in the table in FIG. 3C.

The conventional architecture (external resistors), does not match aswell as this invention, and therefore has worse accuracy. Moreover, thetemperature variation depends on the resistor used and on their positionon the board and therefore is less accurate than in the invention.

This description of the invention has been presented for the purposes ofillustration and description. It is not intended to be exhaustive or tolimit the invention to the precise form described, and manymodifications and variations are possible in light of the teachingabove. The embodiments were chosen and described in order to bestexplain the principles of the invention and its practical applications.This description will enable others skilled in the art to best utilizeand practice the invention in various embodiments and with variousmodifications as are suited to a particular use. The scope of theinvention is defined by the following claims.

The invention claimed is:
 1. A method comprising: providing adigital-to-analog (DAC) converter circuit; providing a first operationalamplifier circuit; coupling a first input of the first operationalamplifier circuit to the digital-to-analog converter circuit; providinga second operational amplifier circuit; coupling a first input of thesecond operational amplifier circuit to an output of the secondoperational amplifier circuit and a voltage output; and coupling a firstinternal impedance between a first supply voltage and a voltage biasnode.
 2. The method of claim 1 comprising: coupling a second internalimpedance between the voltage bias node and a second supply voltageline; coupling the voltage bias node to a second input of the secondoperational amplifier circuit; and coupling a transistor between thevoltage bias node and a second input of the first operational amplifiercircuit.
 3. The method of claim 2 comprising: coupling a third internalimpedance between the second input of the first operational amplifiercircuit and the second supply voltage line.
 4. The method of claim 1wherein the digital-to-analog converter circuit comprises 16 bits orless.
 5. The method of claim 1 wherein the digital-to-analog convertercircuit comprises a number of bits between 3 bits and 16 bits.
 6. Themethod of claim 3 wherein the third internal impedance is electricallyprogrammable in a range from about 1 kiloohm to about 20 kiloohms. 7.The method of claim 1 wherein the first internal impedance has a valueof at least one of about 50 kiloohms or 100 kiloohms.
 8. The method ofclaim 1 wherein the first supply voltage has a value of about 12 volts.9. The method of claim 1 wherein the first internal impedance isprogrammable to have a plurality of impedance values.
 10. The methodclaim 2 wherein the second internal impedance is programmable to have aplurality of impedance values.
 11. The method of claim 3 wherein thethird internal impedance is programmable to have a plurality ofimpedance values.
 12. The method of claim 3 where a connection to thethird internal impedance is not made available via an external pin. 13.The method of claim 1 where a connection to the second input node of thesecond operational amplifier circuit is not made available via anexternal pin.
 14. The method device of claim 1 wherein a connection toan output of the digital-to-analog converter circuit is not madeavailable via an external pin.
 15. The method of claim 1 wherein thefirst input of the first operational amplifier circuit is a positiveinput.
 16. The method of claim 1 wherein the first input of the secondoperational amplifier circuit is a negative input.
 17. A methodcomprising: providing a digital-to-analog converter circuit; providing afirst operational amplifier circuit; coupling a first input of the firstoperational amplifier to the digital-to-analog converter circuit;providing a second operational amplifier circuit; and coupling a firstinput of the second operational amplifier to an output of the secondoperational amplifier circuit and a voltage output.
 18. The method ofclaim 17 comprising: providing a first internal impedance; and couplingthe first internal impedance between a first supply voltage and avoltage bias node; providing a second internal impedance; coupling thesecond internal impedance between the voltage bias node and a secondsupply voltage line; and coupling the voltage bias node a second inputof the second operational amplifier circuit.
 19. The method of claim 18comprising: providing a transistor; coupling the transistor between thevoltage bias node and a second input of the first operational amplifiercircuit; providing a third internal impedance; and coupling the thirdinternal impedance between the second input of the first operationalamplifier circuit and the second supply voltage line.
 20. The method ofclaim 17 wherein the digital-to-analog converter circuit comprises 16bits or less.
 21. The method of claim 17 wherein the digital-to-analogconverter circuit comprises a number of bits between 3 bits and 16 bits.